1. Field of the Invention
The present invention relates to a method for determining a reference position of an arm of an industrial robot to position the arms of the industrial robot at the reference position, and an industrial robot.
2. Description of the Related Art
Generally, so-called articulated robots and other robots are widely used as industrial robots such as welding robots. An articulated robot includes, for example, built-in motor, power transmission mechanism, encoder and the like in each joint, counts pulses supplied from the encoders with reference positions set at “0”, detects rotational positions of arms based on count values and performs a control to drive the arms to desired rotational positions instructed by a program.
When motors for driving arms, power transmission mechanisms for transmitting drive forces of the motors to the arms and the like are incorporated into an articulated robot such as in the case of producing a new robot, relative positions of the arms and relative positions of the arms and the motors or power transmission mechanisms are not specified. This requires an operation of positioning the arms and the like at reference positions.
When a drive motor or power transmission mechanism is replaced such as in the case of removing a motor for driving an arm, a power transmission mechanism for transmitting a drive force of the motor or the like from a robot due to a failure or the like and mounting a new motor or power transmission mechanism, relative positions of arms or relative positions of the arm and the motor or power transmission mechanism may be shifted. Thus, after the replacement of the motor, the power transmission mechanism or the like, an operation of positioning the arms and the like at reference positions is necessary to cause the robot to move as before in accordance with an existing program.
For a positioning operation, there is a method for positioning a tool mounting rotation arm on a swing arm by providing the swing arm with a reference pin, providing the tool mounting rotation arm to be driven and rotated relative to the swing arm with a projection and bringing the projection of the tool mounting rotation arm into contact with the reference pin of the swing arm for adjusting a fifth-axis (A12) reference position of a six-axis articulated robot as disclosed, for example, in Japanese Unexamined Patent Publication No. H06-210586 (hereinafter, referred to as “patent literature 1”). Alternatively, a method for positioning a tool mounting rotation arm on a swing arm using a scribe line formed around a rotation axis of the tool mounting rotation arm has been proposed for a positioning operation.
Further, a reference position determining method for setting a reference position by fixing a jig including a tapered hole to a robot base, fixing a jig to be fitted into the tapered hole formed in the jig fixed to the robot base to a robot wrist (tool mounting portion) and fitting the jig fixed to the robot wrist into the tapered hole of the jig fixed to the robot base as disclosed in Japanese Unexamined Patent Publication No. H10-34572 (hereinafter, referred to as “patent literature 2”) has been proposed as another reference position determining method.
A method for determining a reference position of a tool mounting rotation arm using a level or the like is also used in industrial robots.
FIG. 10 are diagrams showing a conventional method for positioning arms of an industrial robot. FIG. 10A is a plan view of an essential part of the industrial robot, FIG. 10B is a side view of the essential part of the industrial robot in a normal adjusted state and FIG. 10C is a side view of the essential part of the industrial robot in the event of maladjustment.
In the method for determining a reference position of a tool mounting rotation arm using a level or the like, a level 613 is first mounted on a swing arm 611 as shown in FIGS. 10A and 10B and an operator adjusts the position of the swing arm 611 by rotating the swing arm 611 about a motion axis (third axis) A11 thereof so that the swing arm 611 becomes horizontal while watching the level 613. Note that the motion axis (third axis) A11 of the swing arm 611 is a center of rotation of the swing arm 611 and, for example, an axis extending in a direction parallel to a mounting surface of the industrial robot.
Subsequently, the level 613 is mounted on a tool mounting rotation arm 612 and the operator adjusts a reference position of the tool mounting rotation arm 612 by rotating the tool mounting rotation arm 612 about a motion axis (fifth axis) A12 thereof so that the tool mounting rotation arm 612 becomes horizontal while watching the level 613. The motion axis A12 is a center of rotation of the tool mounting rotation arm 612 and an axis parallel to the motion axis A11. At this time, the tool mounting rotation arm 612 is positioned at a reference position when the swing arm 611 is in a horizontal state and the tool mounting rotation arm 612 is in a horizontal state as shown in FIG. 10B.
However, with the reference position determining method disclosed in patent literature 1, positioning accuracy is poor since a movement amount of a positioning portion is small relative to a rotational displacement if positioning is performed in a place where a long distance from a center of rotation of a tip part or the like to the positioning portion cannot be ensured. For example, in a welding robot or the like, a wrist and its vicinity where a welding tool is to be attached is designed to be as small as possible for the purpose of facilitating entrance of the welding tool into a narrow place to be welded. Thus, a long distance from a center of a rotating shaft for positioning to a place where positioning is watched cannot be ensured, wherefore adjustment accuracy cannot be improved.
Further, the reference position determining method disclosed in patent literature 2 requires the jig to be mounted on the robot base and a surrounding area, e.g. a work area becomes narrower due to the jig. Since the robot in a posture for reference position adjustment projects into the work area, the robot cannot take the posture for reference position adjustment at a site of work. Since the reference position is adjusted using a plurality of shafts (two shafts to all shafts), positioning is performed regardless of individual position shifts, wherefore the reference positions of the individual arms may not be correctly adjusted. Further, since many parts are required for reference position determination, installation errors of the parts increase and adjustment accuracy cannot be improved.
Further, the method for mounting the level 613 on the upper surfaces of the swing arm 611 and the tool mounting rotation arm 612 and setting the reference positions in the state where both the swing arm 611 and the tool mounting rotation arm 612 are in the horizontal state as shown in FIG. 10 requires the level 613. To determine reference positions with high accuracy, a high-accuracy level is necessary, but it is expensive. Further, even if the tool mounting rotation arm 612 is horizontal, an error θ of the swing arm 611 is reflected as an error of the tool mounting rotation arm 612 as shown in FIG. 10C and accurate positioning cannot be performed. Further, an adjustment becomes cumbersome.